The term "power distribution substation" is used herein to refer to an electrical substation where incoming high-voltage electrical power, ranging from 60 kV to 245 kV, is converted and distributed to a consumption pool at a voltage equal to or lower than 36 kV. So-called "primary" electrical equipment, including power transformers (both high voltage and mid-voltage), circuit breakers, switches, bus-banks and bus-bars, are employed to effect and condition the voltage reduction that is the primary function of the power substation, and to distribute the reduced-voltage power to a plurality of outgoing lines.
There are several well-known types of power distribution substations. The most common is the "exterior" or "outdoor" substation, in which the primary electrical equipment, including transformers, circuit breakers and associated equipment, is installed in the open air on an outdoor site. Outdoor substations typically employ conventional ambient-air-insulated equipment, including high-voltage circuit breakers and transformers, that is "tried and true" and comparatively inexpensive, and the electrical equipment is usually readily accessible for purposes of maintenance, repair and replacement. However this type of substation has the disadvantages that it generally requires a large surface area, and that it is esthetically unattractive. Another disadvantage of the outdoor substation type is that the electrical equipment is subject to climatic and weather conditions, which may be harsh and include wide temperature ranges, rain, lighting, and possibly icing conditions, which may occasion a great deal of maintenance.
The space requirements of outdoor substations, and their unesthetic appearance, disqualify this type of substations for many urban applications, at a time when urban populations, and their needs for electrical power, have grown rapidly and often require high-voltage feeds, up to 245 kV, into the heart of many cities, for distribution therein at reduced voltages of 36 kV or less.
For urban applications where the space and other conditions required by conventional outdoor substations are not available, a second type of substation, termed "gas-insulated", has been developed and is now widely used. As examplified by the substations disclosed in U.S. Pat. No. 5,777,842 issued to Tsukushi et al., and in U.S. Pat. No. 4,500,935 issued to Tsuruta et al., gas-insulated substations make use of electrical components, including transformers, circuit breakers, bus-banks and switches, which are individually contained in sealed enclosures, and that are insulated within said enclosures by a pressurized dielectric fluid, principally including sulfur hexafluoride (SF.sub.6) gas. Such "gas-insulated" substations are generally much smaller than substations of the "outdoor" type, and their primary electrical equipment is protected from the weather, but this type of substation also has a number of serious disadvantages, which render "gas-insulated" substations unsuitable for many applications, including many "small lot" applications for which outdoor substations are also not feasible.
A first and important disadvantage of "gas-insulated" substations is that the acquisition cost of compact "gas-insulated" electrical components is three to four times the cost of electrically comparable ambient-air-insulated electrical equipment. Thus the acquisition cost of a high-ampere gas-insulated circuit breaker, suitable for handling 145 kV electrical power, is several times the cost of an ambient-air-insulated circuit breaker having comparable performance specifications, and similar cost differences, between ambient-air and gas-insulated versions, apply to other primary electrical components. As a result the total cost of a gas-insulated substation is typically several times the construction cost of a substation that employs ambient-air components to achieve comparable functional performance.
Maintenance and repair costs are also much higher for gas-insulated equipment than for ambient-air insulated equipment. This is in large part because the most widely used insulating fluid, compressed sulfur hexafluoride gas, is a dangerous product with toxic and highly corrosive decomposition products, and it must be handled only by specially qualified personnel, using specialized equipment and acting with great precaution. Furthermore the use of electrical components insulated with sulfur hexafluoride gas is increasingly discouraged, and may in the foreseeable future be banned, because this gas is environmentally very deleterious. As reported in a paper submitted to the 1998 Circuit Breaker Test and Maintenance Conference, held Oct. 6-9, 1998 in Pittsburgh, Pa., the chemical characteristics of sulfur hexafluoride gas, notably its great capacity for absorbing infrared radiation, have "resulted in an extremely high Global Warming Potential rating", equal to 25,000 times the GWP rating of carbon dioxide (the most common greenhouse gas) and qualifying SF.sub.6 as "the most powerful greenhouse gas known." The use and disposal of gas-insulated electrical equipment therefore poses serious environmental hasards, which further disfavors the use of this equipment.
In view of the respective shortcomings of outdoor substations and of gas-insulated substations, described above, there exists a great need for a low-cost and easily serviced indoor substation, that can be located on a small urban lot and that avoids the disadvantages of said prior art substations. Prior art efforts to meet this need, by providing an indoor substation that uses principally ambient-air-insulated electrical equipment, include notably the substation shown in our earlier issued U.S. Pat. No. 5,648,888 to Le Francois et al., issued on Jul. 15, 1997. That substation has the important disadvantage, however, that the high-voltage equipment is arranged on two or more levels of the building, and generally in a manner that makes accessibility to the primary electrical equipment difficult, whether for installation, servicing or removal, resulting in increased operating costs. Also the electrical equipment used in said substation is ill-suited to handle incoming voltages higher than 145 kV, and the dimensions of the substation (about 150 feet square for a single transformer station) are comparatively large, making this substation unsuitable for many urban sites. For these reasons, no presently known substation design provides a fully enclosed, compact power distribution substation, in circumstances where the incoming voltgage is higher than 100 kV. Gas-insulated substations, with all of their disadvantages, as presently the only feasible design solution for such applications.